1. Field of the Invention
The present invention relates to a control technique for an optical modulator used in an optical communication, and particularly, relates to a control technique for compensating for the phase shift between a plurality of drive signals driving the optical modulator and the operating point deviation of the optical modulator.
2. Description of the Related Art
At present, a practical use of optical transmission system in which a transmission speed of optical signals is 10 Gb/s or the like, has bee started. However, due to a recent rapid increase of network utilization, the larger network capacity has been required, and a demand for ultra long distance has been increased.
In an optical transmission system in which a transmission speed is equal to or more than 10 Gb/s, the wavelength dispersion significantly affects waveforms, leading to a wider optical spectrum. As a result, a WDM transmission in which channel lights are arranged in high density becomes difficult. Particularly, in an optical transmission system of 40 Gb/s, the wavelength dispersion is one of factors limiting a transmission distance.
As one means for solving the above described problems, a dispersion compensation technique for accurately measuring a dispersion value of an optical transmission path to compensate for the dispersion value has been studied (refer to Japanese Unexamined Patent Publication No. 11-72761 and Japanese Unexamined Patent Publication No. 2002-077053). Moreover, for realizing the above described optical transmission system, it is essential to develop a modulation system in which the dispersion tolerance is as large as possible. Specifically, a modulation system is required, in which an excellent optical signal to noise ratio can be secured for a long distance optical transmission system, that is to say, a modulation system is required, which is strong in the self phase modulation (SPM) effect and can increase an upper limit of optical input power into the optical transmission path. Furthermore, a modulation system is required, in which the optical spectrum is narrow, to enable a high density WDM optical transmission for the large capacity.
Recently, as new modulation systems, a Carrier-Suppressed Return-to-Zero (hereunder, CS-RZ) modulation system and the like have been studied (refer to Y. Miyamoto et. al., “320 Gbit/s (8×40 Gbit/s) WDM transmission over 367-km zero-dispersion-flattened line with 120-km repeater spacing using carrier-suppressed return-to-zero pulse format”, OAA'99 PD, PdP4). Since this CS-RZ modulation system has an advantage that, as described later, the optical spectrum width becomes ⅔ times compared to the Return-to-Zero (RZ) modulation system, the wavelength dispersion tolerance is large, which enables a high density channel light arrangement in the WDM. Furthermore, since the waveform deterioration due to the self phase modulation effect is small, it becomes possible to secure the optical signal to noise ratio for the long distance transmission.
FIG. 25 is a diagram showing a basic configuration for generating a CS-RZ modulating signal of 40 Gb/s.
In FIG. 25, a light source 100 generates a continuous light. The continuous light output from the light source 100 is sequentially input to two LiNbO3 modulators (hereunder, LN modulators) 110 and 120 connected in series, to be modulated.
The former stage LN modulator 110 is applied with, at a signal electrode thereof (not shown in the figure), for example, a data signal with bit rate of 40 Gb/s, which is generated in a data signal generating section 111 and corresponds to the NRZ modulation system, as a drive signal. As a result, the former stage LN modulator 110 modulates the continuous light from the light source 100 in accordance with the data signal, and outputs an NRZ signal light of 40 Gb/s having a waveform as exemplified in (a) of FIG. 26 to the latter stage LN modulator 120.
For the latter stage LN modulator 120, for example, a Mach-Zehnder (MZ) modulator or the like having two signal electrodes is used. The latter stage LN modulator 120 is applied with, at the respective signal electrodes thereof, a first drive signal and a second drive signal generated based on a clock signal having a frequency of ½ the bit rate of the data signal. As a result, the latter stage LN modulator further modulates the NRZ signal light from the former stage LN modulator 110, and outputs a CS-RZ signal light of 40 Gb/s having a waveform as exemplified in (b) of FIG. 26. Here, a clock signal having a waveform of a sine wave and the like with frequency 20 GHz, is generated in a clock signal generating section 121. The clock signal is branched into two by a branching device 124, and then adjusted by phase shifters 125A and 125B so that a phase difference between branched signals reaches approximately 180°. Furthermore, respective amplitudes of the branched signals are adjusted by amplifiers 126A and 126B, to become first and second drive signals to be applied to the respective signal electrodes of the LN modulator 120.
Moreover, a part of the clock signal generated in the clock signal generating section 121 is branched by a branching device 122 and transmitted to the data signal generating section 111 so that phases of the data signal and clock signal are synchronized, and at the same time, the phase of the clock signal is adjusted by a phase shifter 123 so that a phase difference between the respective signals is controlled.
Here, the theory of how the CS-RZ signal light of 40 Gb/s is generated is briefly described using an optical intensity characteristic of an LN modulator to a drive voltage, shown in FIG. 27.
Generally, in the case where a signal light corresponding to the NRZ modulation system or the RZ modulation system is generated using an optical modulator, an optical intensity characteristic of which to a drive voltage is changed periodically, a drive voltage corresponding to adjacent “top, bottom” or “bottom, top” of the above optical intensity characteristic (hereunder, this drive voltage is Vπ) is given to the optical modulator, so as to perform the modulation. Here, “top” of the optical intensity characteristic denotes emission peak points and “bottom” denotes extinction peak points.
On the other hand, in the case where a signal light corresponding to the CS-RZ modulation system is generated, the signal light of 40 Gb/s modulated by the former stage LN modulator 110 shown in FIG. 25 in accordance with the data signal, is further modulated by the latter stage LN modulator 120 in accordance with the clock signal of 20 GHz having the frequency of ½ the bit rate of the data signal. The latter stage LN modulator 120, as shown in the left of FIG. 27, is applied with a drive voltage corresponding to “top, bottom, top” of the optical intensity characteristic to the drive voltage (hereunder, this drive voltage is 2Vπ). This light modulation is performed by corresponding the respective levels of −1, 0, 1 of the clock signal to the respective conditions of ON, OFF, ON of the light. As a result, the CS-RZ signal light generated becomes a binary optical waveform as shown at the top right of FIG. 27. For the signal light in this CS-RZ modulation system, since optical phases of respective bits thereof have a value of 0 or π, for example, as shown in a calculation result of the optical spectrum at the bottom right of FIG. 27, a carrier component of the optical spectrum is suppressed.
For the signal light of the CS-RZ modulation system generated as described above, for example, as in the respective experimental results of the optical spectrum and optical waveform shown in FIG. 28, an optical waveform of the form approximately the same as the optical waveform of the RZ modulation system can be obtained, and the optical spectrum width becomes narrower than that of the RZ modulation system. Moreover, as in the experimental results related to the wavelength dispersion tolerance shown in FIG. 29, a range of total wavelength dispersion where a value of power penalty becomes equal to or less than 1 dB, is approximately 40 ps/nm in the RZ modulation system, whereas in the CS-RZ modulation system, the range is approximately 50 ps/nm. Accordingly, it is understood that, for the signal light of the CS-RZ modulation system, the dispersion tolerance is enlarged compared to the signal light of the RZ modulation system.
Incidentally, the signal light corresponding to the CS-RZ modulation system has the above described advantages, but there are disadvantages in that; the phase between the first and second drive signals to be given to the latter stage optical modulator which is driven based on the clock signal, should be precisely adjusted, and the phase between the above described clock signal and the data signal used for driving the former stage optical modulator should also be precisely adjusted. Furthermore, since there is a possibility that the phase shift occurs due to environmental changes such as temperature changes, it becomes essential to detect a phase change in each signal during the system operation, to perform a feedback control.
Here, the present applicant has proposed a system, for example as shown in FIG. 30, for monitoring the optical spectrum of signal light output from an optical modulator by a monitoring section 130, and then based on an intensity variation of a specific frequency component in the optical spectrum, feedback controlling the above described phase shift between drive signals by a control circuit 140 (refer to Japanese Patent Application No. 2002-087017). According to this prior invention, by focusing on the intensity variation of the specific frequency component of the output optical spectrum, the phase shift between signals of the drive system can be reliably detected, and it becomes possible to control the phase difference between drive signals so that an optimum drive condition can be obtained stably.
However, the control system of the optical modulator according to this prior invention has the following problems. That is, in the above described control method, the specific frequency component of the output optical spectrum is extracted using a narrow-band optical filter 132, to monitor the intensity variation. However, at this time, there is a problem that, unless the specific frequency component is extracted using an optical filter with a sufficiently narrow bandwidth of transmission band, the monitoring accuracy of the intensity variation is reduced. Generally, an optical filter having a sufficiently narrow bandwidth is not easily realized. Therefore, due to the reduction of monitoring accuracy of the intensity variation as described above, there is a possibility that it becomes difficult to feedback control stably the phase difference between the drive signals. Furthermore, for the control system of the prior invention, another problem is that a control corresponding to the operating point variation of the optical modulator has not yet been realized.